This invention relates to acoustic transducers, for use such as in loudspeakers, headphones and microphones, and in particular to such acoustic transducers based upon electrostatic principles.
It is well known to produce an acoustic transducer based upon electrostatic principles. Generally, as shown in Lange, U.S. Pat. No. 3,362,903, an electrostatic acoustic transducer includes a pair of fixed electrodes, often with a multiplicity of small openings and supported opposite to each other, and with a vibrating film or diaphragm affixed therebetween. The diaphragm is caused to vibrate according to the desired acoustical frequencies by applying electrical fields to the electrodes, so as to faithfully reproduce an audio signal.
Often the material of the diaphragm is a polyethylene terephthalate film manufactured by DUPONT under the trademark MYLAR. As is well known, MYLAR.TM. has an extremely high sheet resistivity, on the order of about 10.sup.18 ohms per square, which causes the attendant electronic drive circuitry to be exceedingly complex and expensive It may be that a coating of a complex quaternary compound used in the fabric industry could be applied to a MYLAR.TM. diaphragm to reduce the resistivity. When such quaternary compounds are used, however, the sheet resistivity of the surface is dependent upon ambient humidity, and can change noticeably with small changes in humidity, adversely affecting the acoustical performance of any transducer of which it is a part. These changes based upon humidity are particularly noticeable if the acoustic transducer is used in a headphone speaker application, because of the increased humidity caused by the proximity of the transducer to the listener's head and ear.
As indicated in Atoji, U.S. Pat. No. 3,833,770, it is known to provide an electret, the electrostatic equivalent of a permanent magnet, with a conductive layer formed on one side thereof. Atoji proposes to have the electret generate an electrostatic field between itself and one of the fixed electrodes and thereby eliminate certain harmonics generated between the electret and the other fixed electrode. Thus, Atoji proposes to apply highly conductive materials such as aluminum or silver to one side of the diaphragm. This application of highly conductive materials to the diaphragm material, however, drastically reduces the sheet resistivity of the diaphragm, and adds significant mechanical mass to the diaphragm. The overall effect of these changes is to reduce the acoustic performance of the transducer, unless other changes are made in the design and construction of the transducer to maintain sound quality. Such changes to maintain sound quality again result in increased expense of manufacture.
This invention relates to improvements over the structures described above, and to solutions to some of the problems raised thereby.